Ashar,
you didn't mention speed of fiber thread.Very often in this industial field there is hightech machinery and very high speed.Also ceramic components are employed to
resist for a reasonble period.Abrasion is inevitable...these machines work 24h a day.
My suggestion would be a hanging lever that is deflected by the fiber sidewards.If there is no (more)
fiber the lever moves to vertical by gravity and closes a microswitch or inductive sensor.
This solution requires no tricks to get the thread into position ,it is low cost,has no optical parts and is certainly easy to build and mount onto the machines.
Edgar